The "Indy 500" is a nickname for the Indianapolis 500. It is a car race. Indianapolis is a city, the capital city of the state of Indiana. The Indianapolis Motor Speedway is a 2.5-mile long oval car racetrack. The race is called the Indianapolis 500 because it is 500 miles (804 km) long. It is run in special IndyCars built only for racing; they use special fuels, special engines, and so forth. The Indy 500 is held on Memorial Day weekend. It is currently held on the day before Memorial Day (i.e. on Sunday rather than Monday). It is not any official part of the Memorial Day observances, it's just "something that happens on Memorial Day weekend." Memorial Day is held on the last Monday in May, and is held in memory of people who died while serving in the armed forces of the United States. The chief remembrance event on Memorial Day is putting flowers on the graves of those who fell. Because Memorial Day creates a three-day weekend for most people, it is a good time for scheduling special weekend events. Memorial Day is often regarded as the "unofficial beginning of summer;" for example, resorts that are open for "the summer" will typically open for Memorial Day weekend and close after Labor Day weekend in September. So the Indy 500 is just part of "something that happen at the beginning of summer."

It is simply a coincidence - but a great day to have the Indy 500 because people are celebrating Memorial day - they have free time to attend the Indy 500 or seeit on TV.